1. Field of the Invention
The present invention relates to a technique, for measuring the speed of a vehicle, using radio-waves. More particularly, it relates to a non-contact speed measuring apparatus which is provided on a railroad vehicle and measures a speed of the vehicle so as to determine the movement distance and position thereof.
2. Description of the Related Art
As an example of the prior art by which the speed of a railroad vehicle is measured, a system using the output of a tachogenerator, which functions in connection with the rotation of a wheel, is known.
In such a system, the tachogenerator is coupled to an axle of the wheel, so as to output a plurality of alternating current signals per one rotation of the wheel. According to the system, since the diameter of the wheel is known, it is possible to measure the speed of the railroad vehicle and also to obtain information on the movement distance thereof.
However, the system poses a problem in that, where the speed of the vehicle is low, the ability of the tachogenerator is lowered and thus it is difficult to separate output components of the tachogenerator from noise components. This leads to a reduction in the reliability in the speed measurement. Also, where the friction force between the wheel and the rail is low, due to a rain or the like, a slip or glide of the wheel may occur, and thus the tachogenerator cannot carry out its normal operation. This results in a problem in that it is impossible to obtain a unified correlation between the movement distance of the vehicle and the speed thereof, and thus it is impossible to specify the real movement distance and position of the railroad vehicle.
As another example of the prior art by which a speed measurement of a railroad vehicle is carried out, a system using a frequency-modulated continuous wave (FM-CW system) is known.
For example, using a speed measuring apparatus provided on a railroad vehicle, a measurement of the speed of the railroad vehicle is carried out as follows. First, the apparatus transmits a frequency-modulated continuous wave (FM-CW) signal as a radio-wave from an antenna toward a corresponding track, and mixes a signal reflected from the track (i.e., receiving signal) with the transmitted signal. The apparatus then extracts from the mixed signal a signal containing a Doppler frequency component based on the speed relative to the track, and detects the Doppler frequency component to thereby measure the speed of the vehicle.
The FM-CW system is a non-contact system using radio-waves, and thus it is not subject to influences by a slip or glide of wheels as seen in the prior art system using a tachogenerator.
However, the FM-CW system poses a problem in that, where the reflection surface for propagation of the transmitted radio-wave lies in the close neighborhood of the transmission point, the measuring apparatus is subject to the influence by the reflection in the close neighborhood, and/or an influence by interference with the transmission signal or an interference signal in the receiving system or channel.
Also, where the reflection surface for propagation of the transmitted radio-wave lies far away from the transmission point, the influence by the spread of the radio-wave beam radiated from the antenna is averaged, and thus the center of power in the frequency band of the receiving signal can be uniquely determined. However, where the reflection surface lies in the close neighborhood, a problem occurs in that it is impossible to uniquely determine the center of power in the frequency band of the receiving signal since conditions of the reflection surface (e.g., the ground) cannot be ignored. This leads to a difficulty in a precise speed measurement with respect to the railroad vehicle.